WASHINGTON (REUTERS) - Novavax Inc said on Monday (April 5) participants in its ongoing Covid-19 vaccine trials in South Africa and Britain can now receive additional shots, ensuring those who received a placebo can also get the active vaccine.

Allowing volunteers to crossover to the active vaccine offers them an incentive to continue in the trials even when other authorised vaccines are available.

The company said it planned to cross over participants in its ongoing large, late-stage study in the United States and Mexico, for which the company plans to read out initial clinical data during the second quarter.

If authorised for emergency use in the United States, Novavax's two-shot vaccine would help ramp up ongoing vaccination efforts in the country with currently authorised vaccines from Pfizer/BioNTech , Moderna Inc and Johnson & Johnson.

As part of the updated trial design, all participants of the trials in the UK and the US will be offered the chance to receive an additional round of shots, the company said.

Participants who opt to do so will receive two additional doses of either vaccine for those who initially received placebo, or placebo for those who first got vaccine.

All participants will continue to be "blinded" or not know if they got the experimental vaccine or a placebo.